//===- llvm/Analysis/LoopCacheAnalysis.h ------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file defines the interface for the loop cache analysis.
///
//===----------------------------------------------------------------------===//
#ifndef LLVM_ANALYSIS_LOOPCACHEANALYSIS_H
#define LLVM_ANALYSIS_LOOPCACHEANALYSIS_H
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/IR/PassManager.h"
#include <optional>
namespace llvm {
class AAResults;
class DependenceInfo;
class Instruction;
class LPMUpdater;
class raw_ostream;
class LoopInfo;
class Loop;
class ScalarEvolution;
class SCEV;
class TargetTransformInfo;
using CacheCostTy = int64_t;
using LoopVectorTy = SmallVector<Loop *, 8>;
/// Represents a memory reference as a base pointer and a set of indexing
/// operations. For example given the array reference A[i][2j+1][3k+2] in a
/// 3-dim loop nest:
/// for(i=0;i<n;++i)
/// for(j=0;j<m;++j)
/// for(k=0;k<o;++k)
/// ... A[i][2j+1][3k+2] ...
/// We expect:
/// BasePointer -> A
/// Subscripts -> [{0,+,1}<%for.i>][{1,+,2}<%for.j>][{2,+,3}<%for.k>]
/// Sizes -> [m][o][4]
class IndexedReference {
friend raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R);
public:
/// Construct an indexed reference given a \p StoreOrLoadInst instruction.
IndexedReference(Instruction &StoreOrLoadInst, const LoopInfo &LI,
ScalarEvolution &SE);
bool isValid() const { return IsValid; }
const SCEV *getBasePointer() const { return BasePointer; }
size_t getNumSubscripts() const { return Subscripts.size(); }
const SCEV *getSubscript(unsigned SubNum) const {
assert(SubNum < getNumSubscripts() && "Invalid subscript number");
return Subscripts[SubNum];
}
const SCEV *getFirstSubscript() const {
assert(!Subscripts.empty() && "Expecting non-empty container");
return Subscripts.front();
}
const SCEV *getLastSubscript() const {
assert(!Subscripts.empty() && "Expecting non-empty container");
return Subscripts.back();
}
/// Return true/false if the current object and the indexed reference \p Other
/// are/aren't in the same cache line of size \p CLS. Two references are in
/// the same chace line iff the distance between them in the innermost
/// dimension is less than the cache line size. Return std::nullopt if unsure.
std::optional<bool> hasSpacialReuse(const IndexedReference &Other,
unsigned CLS, AAResults &AA) const;
/// Return true if the current object and the indexed reference \p Other
/// have distance smaller than \p MaxDistance in the dimension associated with
/// the given loop \p L. Return false if the distance is not smaller than \p
/// MaxDistance and std::nullopt if unsure.
std::optional<bool> hasTemporalReuse(const IndexedReference &Other,
unsigned MaxDistance, const Loop &L,
DependenceInfo &DI, AAResults &AA) const;
/// Compute the cost of the reference w.r.t. the given loop \p L when it is
/// considered in the innermost position in the loop nest.
/// The cost is defined as:
/// - equal to one if the reference is loop invariant, or
/// - equal to '(TripCount * stride) / cache_line_size' if:
/// + the reference stride is less than the cache line size, and
/// + the coefficient of this loop's index variable used in all other
/// subscripts is zero
/// - or otherwise equal to 'TripCount'.
CacheCostTy computeRefCost(const Loop &L, unsigned CLS) const;
private:
/// Attempt to delinearize the indexed reference.
bool delinearize(const LoopInfo &LI);
/// Attempt to delinearize \p AccessFn for fixed-size arrays.
bool tryDelinearizeFixedSize(const SCEV *AccessFn,
SmallVectorImpl<const SCEV *> &Subscripts);
/// Return true if the index reference is invariant with respect to loop \p L.
bool isLoopInvariant(const Loop &L) const;
/// Return true if the indexed reference is 'consecutive' in loop \p L.
/// An indexed reference is 'consecutive' if the only coefficient that uses
/// the loop induction variable is the rightmost one, and the access stride is
/// smaller than the cache line size \p CLS. Provide a valid \p Stride value
/// if the indexed reference is 'consecutive'.
bool isConsecutive(const Loop &L, const SCEV *&Stride, unsigned CLS) const;
/// Retrieve the index of the subscript corresponding to the given loop \p
/// L. Return a zero-based positive index if the subscript index is
/// succesfully located and a negative value otherwise. For example given the
/// indexed reference 'A[i][2j+1][3k+2]', the call
/// 'getSubscriptIndex(loop-k)' would return value 2.
int getSubscriptIndex(const Loop &L) const;
/// Return the coefficient used in the rightmost dimension.
const SCEV *getLastCoefficient() const;
/// Return true if the coefficient corresponding to induction variable of
/// loop \p L in the given \p Subscript is zero or is loop invariant in \p L.
bool isCoeffForLoopZeroOrInvariant(const SCEV &Subscript,
const Loop &L) const;
/// Verify that the given \p Subscript is 'well formed' (must be a simple add
/// recurrence).
bool isSimpleAddRecurrence(const SCEV &Subscript, const Loop &L) const;
/// Return true if the given reference \p Other is definetely aliased with
/// the indexed reference represented by this class.
bool isAliased(const IndexedReference &Other, AAResults &AA) const;
private:
/// True if the reference can be delinearized, false otherwise.
bool IsValid = false;
/// Represent the memory reference instruction.
Instruction &StoreOrLoadInst;
/// The base pointer of the memory reference.
const SCEV *BasePointer = nullptr;
/// The subscript (indexes) of the memory reference.
SmallVector<const SCEV *, 3> Subscripts;
/// The dimensions of the memory reference.
SmallVector<const SCEV *, 3> Sizes;
ScalarEvolution &SE;
};
/// A reference group represents a set of memory references that exhibit
/// temporal or spacial reuse. Two references belong to the same
/// reference group with respect to a inner loop L iff:
/// 1. they have a loop independent dependency, or
/// 2. they have a loop carried dependence with a small dependence distance
/// (e.g. less than 2) carried by the inner loop, or
/// 3. they refer to the same array, and the subscript in their innermost
/// dimension is less than or equal to 'd' (where 'd' is less than the cache
/// line size)
///
/// Intuitively a reference group represents memory references that access
/// the same cache line. Conditions 1,2 above account for temporal reuse, while
/// contition 3 accounts for spacial reuse.
using ReferenceGroupTy = SmallVector<std::unique_ptr<IndexedReference>, 8>;
using ReferenceGroupsTy = SmallVector<ReferenceGroupTy, 8>;
/// \c CacheCost represents the estimated cost of a inner loop as the number of
/// cache lines used by the memory references it contains.
/// The 'cache cost' of a loop 'L' in a loop nest 'LN' is computed as the sum of
/// the cache costs of all of its reference groups when the loop is considered
/// to be in the innermost position in the nest.
/// A reference group represents memory references that fall into the same cache
/// line. Each reference group is analysed with respect to the innermost loop in
/// a loop nest. The cost of a reference is defined as follow:
/// - one if it is loop invariant w.r.t the innermost loop,
/// - equal to the loop trip count divided by the cache line times the
/// reference stride if the reference stride is less than the cache line
/// size (CLS), and the coefficient of this loop's index variable used in all
/// other subscripts is zero (e.g. RefCost = TripCount/(CLS/RefStride))
/// - equal to the innermost loop trip count if the reference stride is greater
/// or equal to the cache line size CLS.
class CacheCost {
friend raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC);
using LoopTripCountTy = std::pair<const Loop *, unsigned>;
using LoopCacheCostTy = std::pair<const Loop *, CacheCostTy>;
public:
static CacheCostTy constexpr InvalidCost = -1;
/// Construct a CacheCost object for the loop nest described by \p Loops.
/// The optional parameter \p TRT can be used to specify the max. distance
/// between array elements accessed in a loop so that the elements are
/// classified to have temporal reuse.
CacheCost(const LoopVectorTy &Loops, const LoopInfo &LI, ScalarEvolution &SE,
TargetTransformInfo &TTI, AAResults &AA, DependenceInfo &DI,
std::optional<unsigned> TRT = std::nullopt);
/// Create a CacheCost for the loop nest rooted by \p Root.
/// The optional parameter \p TRT can be used to specify the max. distance
/// between array elements accessed in a loop so that the elements are
/// classified to have temporal reuse.
static std::unique_ptr<CacheCost>
getCacheCost(Loop &Root, LoopStandardAnalysisResults &AR, DependenceInfo &DI,
std::optional<unsigned> TRT = std::nullopt);
/// Return the estimated cost of loop \p L if the given loop is part of the
/// loop nest associated with this object. Return -1 otherwise.
CacheCostTy getLoopCost(const Loop &L) const {
auto IT = llvm::find_if(LoopCosts, [&L](const LoopCacheCostTy &LCC) {
return LCC.first == &L;
});
return (IT != LoopCosts.end()) ? (*IT).second : -1;
}
/// Return the estimated ordered loop costs.
ArrayRef<LoopCacheCostTy> getLoopCosts() const { return LoopCosts; }
private:
/// Calculate the cache footprint of each loop in the nest (when it is
/// considered to be in the innermost position).
void calculateCacheFootprint();
/// Partition store/load instructions in the loop nest into reference groups.
/// Two or more memory accesses belong in the same reference group if they
/// share the same cache line.
bool populateReferenceGroups(ReferenceGroupsTy &RefGroups) const;
/// Calculate the cost of the given loop \p L assuming it is the innermost
/// loop in nest.
CacheCostTy computeLoopCacheCost(const Loop &L,
const ReferenceGroupsTy &RefGroups) const;
/// Compute the cost of a representative reference in reference group \p RG
/// when the given loop \p L is considered as the innermost loop in the nest.
/// The computed cost is an estimate for the number of cache lines used by the
/// reference group. The representative reference cost is defined as:
/// - equal to one if the reference is loop invariant, or
/// - equal to '(TripCount * stride) / cache_line_size' if (a) loop \p L's
/// induction variable is used only in the reference subscript associated
/// with loop \p L, and (b) the reference stride is less than the cache
/// line size, or
/// - TripCount otherwise
CacheCostTy computeRefGroupCacheCost(const ReferenceGroupTy &RG,
const Loop &L) const;
/// Sort the LoopCosts vector by decreasing cache cost.
void sortLoopCosts() {
stable_sort(LoopCosts,
[](const LoopCacheCostTy &A, const LoopCacheCostTy &B) {
return A.second > B.second;
});
}
private:
/// Loops in the loop nest associated with this object.
LoopVectorTy Loops;
/// Trip counts for the loops in the loop nest associated with this object.
SmallVector<LoopTripCountTy, 3> TripCounts;
/// Cache costs for the loops in the loop nest associated with this object.
SmallVector<LoopCacheCostTy, 3> LoopCosts;
/// The max. distance between array elements accessed in a loop so that the
/// elements are classified to have temporal reuse.
std::optional<unsigned> TRT;
const LoopInfo &LI;
ScalarEvolution &SE;
TargetTransformInfo &TTI;
AAResults &AA;
DependenceInfo &DI;
};
raw_ostream &operator<<(raw_ostream &OS, const IndexedReference &R);
raw_ostream &operator<<(raw_ostream &OS, const CacheCost &CC);
/// Printer pass for the \c CacheCost results.
class LoopCachePrinterPass : public PassInfoMixin<LoopCachePrinterPass> {
raw_ostream &OS;
public:
explicit LoopCachePrinterPass(raw_ostream &OS) : OS(OS) {}
PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR, LPMUpdater &U);
};
} // namespace llvm
#endif // LLVM_ANALYSIS_LOOPCACHEANALYSIS_H